1. Field of the Invention
The present invention relates to a signal communication device and, more particularly, to a signal communication device operable both in a code division multiple access (CDMA) mode and in a frequency modulation (FM) mode.
2. Description of the Related Art
Typical of automobile and portable telephone systems are frequency division multiple access (FDMA) system and a code division multiple access (CDMA) system. The FDMA system assigns a single channel to a single frequency band width. For example, the American standard advanced mobile phone system (AMPS), which is implemented by the FDMA system, assigns a frequency band of 30 kHz to a single telephone channel.
A current trend in radio telecommunications is toward a digital cellular system, as distinguished from the conventional AMPS or similar analog cellular system. A digital cellular system is advantageous over the analog system in that it accommodates a greater number of subscribers, insures stable sound quality, has a secrecy function, and matches other digital services. Particularly, a CDMA system proposed by Qualcomm scatters 10 kHz coded voice data to a frequency band of 1.25 MHz and allows a plurality of conversation channels to share the same frequency band. This CDMA system is a promising system since it insures a frequency utilization efficiency ten times to twenty times that of the AMPS system.
Such a CDMA system is disclosed, for example, in a paper entitled "TIA/SIA INTERIM STANDARD" by TELECOMMUNICATIONS INDUSTRY ASSOCIATION, pages 6-1 to 7-42, and published in July 1993.
In North America, however, it is a prerequisite that a new digital system be introduced together with an apparatus also operable in the conventional AMPS system, so-called dual mode. Therefore, a receiver circuit built in a cellular telephone has to be adaptive to both of AMPS and CDMA.
To better understand the present invention, a brief reference will be made to a prior art receiver of the AMPS cellular terminal unit, shown in FIG. 1. The receiver is implemented as a double superheterodyne receiver.
The receiver includes an antenna 1, a band-pass filter 2 for filtering a high frequency signal coming in through the antenna 1, a high frequency amplifier 3, a first mixer 4 for combining the output of the amplifier 3 with a first local oscillation signal to produce a first intermediate frequency (IF) signal, a first IF filter 6 for filtering the first IF signal, a first IF amplifier 7 for amplifying the output of the IF filter 6, and a second mixer 8 for combining the output of the IF amplifier 7 with a second local oscillation signal to thereby output a second IF signal.
In operation, the antenna 1 receives a high frequency signal lying in the range of from 869.04 MHz to 893.97 MHz. The bandpass filter, or high frequency filter, 2 supplies a signal component lying in the above-mentioned range. The high frequency amplifier, or low noise amplifier (LNA), 3 amplifies the filtered signal component. The first mixer 4 mixes the output of the amplifier 3 with a first local oscillation signal, thereby producing a first IF signal. The first local oscillation signal is generated by a first local oscillator 5. Usually, the local oscillator 5 is implemented by a phase locked loop (PLL) synthesizer in order to change the frequency on a 30 kHz basis for channel selection. The first IF signal usually lies in a frequency band of 70 MHz or 90 MHz.
The first IF signal is further filtered by the first IF filter 6 so as to have spurious components thereof removed. The first IF amplifier 7 amplifies the filtered signal of the IF filter 6. The second mixer 8 mixes the amplified signal of the amplifier 7 with a second local oscillation signal to output a second IF signal whose frequency is usually about 450 kHz. The second local oscillation signal is generated by a second local oscillator 9. The second IF filter 10 filters the second IF signal for thereby removing spurious waves farther than the adjoining channels. A limiter amplifier 11 amplifies the output signal of the second IF filter 10 to saturation, thereby removing varying amplitude components. The output of the limiter amplifier 11 is applied to a frequency discriminator 12 with the result that voice, or FM signal, is reproduced. Although the limiter amplifier 11 removes amplitude components from the second IF signal, it outputs a voltage corresponding to a field strength, i.e., received signal strength indicator (RSSI). The receiver senses a received field strength by use of the RSSI signal. The second IF filter 10, limiter amplifier 11 and frequency discriminator 12, among others, are extensively used and available at extremely low cost.
Analog-to-digital converters (A/D) 13 and 14 convert the voice signal and RSSI signal to digital signals, respectively, and supply the digital signals to a digital signal processor (DSP) 15.
The circuitry shown in FIG. 1 cannot accommodate the CDMA system for the following reasons. As for the AMPS system, the band width per channel is less than 30 kHz and can be easily dealt with by the second IF filter whose frequency band is 450 kHz. However, when it comes to the CDMA system, the band width is as broad as about 1.2 MHz and cannot be handled by the second IF filter.
Furthermore, the AMPS system is an FM system and the received signal does not contain any amplitude information thereof. This allows an inexpensive limiter amplifier to be used. By contrast, the CDMA system is a four-phase phase shift keying (PSK) modulation system and the signal contains information even in amplitude modulation. Therefore, the inexpensive limiter amplifier cannot be used in the CDMA system. In addition, nonlinear processing is not available with the CDMA system since telephone signals of as many as about sixty subscribers exist together on a signal frequency channel.